Over the last few decades, the electronics industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly integrated electronic devices. The most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applicabilities and numerous disciplines. An example of such a silicon-based semiconductor device is a metal-oxide-semiconductor (MOS) transistor. The principle elements of a typical MOS semiconductor device are illustrated in FIG. 1. The device generally includes a gate electrode 101, which acts as a conductor, to which an input signal typically is applied via a gate terminal (not shown). Heavily doped source region 103 and drain region 105 are formed in a semiconductor substrate 107, and respectively are connected to source and drain terminals (not shown).
A channel region 109 is formed in the semiconductor substrate 107 beneath the gate electrode 101 and separates the source region 103 and drain region 105. The channel typically is lightly doped with a dopant of a type opposite to that of the source and drain regions. The gate electrode 101 is physically separated from the semiconductor substrate 107 by a gate insulating layer 111. Typically, this insulating layer is an oxide layer such as SiO.sub.2. The insulating layer 111 is provided to prevent current from flowing between the gate electrode 101 and the semiconductor source region 103, drain region 105 or channel region 109.
In operation, an output voltage typically is developed between the source and drain terminals. When an input voltage is applied to the gate electrode 101, a transverse electric field is set up in the channel region 109. By varying the transverse electric field, it is possible to modulate the conductance of the channel region 109 between the source region 103 and drain region 105. In this manner, an electric field controls the current flow through the channel region 109. This type of device commonly is referred to as a MOS field-effect transistor (MOSFET). Semiconductor devices such as the one described are used in large numbers to construct most modern electronic devices. In order to increase the capability of such electronic devices, it is necessary to integrate ever larger numbers of such devices into a single silicon wafer. As the semiconductor devices are scaled down in order to form a larger number of such devices on a given surface area, the structure of the devices and the fabrication techniques used to make the devices must be altered.
One important property of MOS devices is drive current strength. It is particularly important to provide semiconductor devices that exhibit the designed drive current strength and to maintain a substantially constant drive current strength between production lots and within a single production lot.
The drive current strength is proportional to capacitance, which in turn is inversely proportional to the thickness of the gate insulating layer. The drive current strength also is inversely proportional to channel length. As the dimensions of semiconductor devices become smaller and smaller, production variations in gate insulating layer thickness have an increased significance with respect to variation in drive current strength.